For many years, various materials which contain lead have come in contact with foods and liquids intended for human consumption. Such lead-containing materials include brass alloys made of copper, lead, and zinc used for plumbing fittings; bronze alloys; lead solder in pipes; lead-containing compounds used in tins for storing food items such as olive oil; etc.
Brass alloys used to manufacture fittings such as faucets and valves are made up primarily of copper and zinc, with a small amount of lead added to make the brass more workable and machinable. Easier machinability permits finishing, machining, the cutting of threads, etc., to proceed more smoothly and at a lower cost than without a lead alloying moiety. Bronze alloys are similarly made up primarily of copper and tin, with a small amount of a lead alloying moiety for similar reasons.
Because lead atoms are much larger than copper and zinc atoms, the lead atoms have very low solid solubility in brass alloys. The lead atoms therefore tend to precipitate as lead-rich pockets dispersed through the brass. Surfaces of brass fixtures generally have lead concentrations much higher than the average concentration of lead throughout the fixtures. These lead-rich pockets improve the machinability of the bronze. However, they also increase the tendency of lead to leach into water.
Until recently, the amount of lead leached into foods and liquids from modern lead-containing plumbing fittings was considered to be low enough that it presented no significant harm to ingesters of such foods and liquids. However, new, stricter standards which significantly limit the amount of permitted lead leaching and lead exposure are being promulgated and imposed at both the state and federal levels. For example, the Safe Drinking Water Act was amended in June 1988 to limit lead in solders and fluxes to 0.2 percent and to limit lead in public water supply pipe and fittings to eight percent. Lead soldered food cans have not been made in the United States since 1991. Regulations such as these limit lead exposure by limiting the amounts of lead in materials in contact with foods and liquids.
Another approach to limiting lead exposure is to limit the amount of lead which is actually in the food or water. For example, regulations implementing California's Safe Drinking Water and Toxic Enforcement Act of 1986 limit lead exposure of an individual to less than 0.5 microgram per day. In 1991, the EPA increased the stringency of the lead standard for drinking water from 50 parts per billion to 15 parts per billion. In December 1994, a consortium led by NSF International developed a voluntary third-party consensus standard, NSF Standard 61, Section 9-1994, and a certification program for all direct and indirect drinking water additives. Among these standards is one for lead, which limits the amount of lead from most endpoint devices to 11 micrograms (.mu.g) when normalized for the one liter first draw sample.
Although the amount of lead leached from brass alloy faucets, valves and other plumbing fittings and fixtures made using current methods is low, the amount of lead leached from such fittings may exceed current or planned permissible standards. Such more stringent standards require either that lead be entirely eliminated from the brass alloys or that the brass be treated so that lead does not leach out in amounts which exceed permitted standards. Treatment of a material to reduce its chemical activity is sometimes referred to as passivation.
Previous lead control strategies recommended in the Lead and Copper Rule, 40 CFR .sctn..sctn.141-142 (U.S. EPA 1991) have focused on water stabilization and corrosion inhibition. These treatments do not remove lead, but merely precipitate it or change its oxidation kinetics.
To stabilize water, its pH is adjusted, using, for example, lime (CaO), slaked lime (Ca(OH).sub.2), and caustics (NaOH, KOH). Alternatively, the alkalinity of water is adjusted, using, for example, sodium bicarbonate, sodium carbonate, and sodium silicate.
To inhibit corrosion, various inorganic phosphate salts and sodium silicate may be added. Zinc and other orthophosphates, sodium pyrophosphate, and sodium tripolyphosphate have been used. Phosphate treatment is not effective in low pH water. Polyphosphates apparently contain or convert to orthophosphates which form metal orthophosphate films on plumbing materials. Although sodium silicate inhibits corrosion of galvanized steel and copper-based metals by forming metal silicate films, it requires high doses and months of treatment to be effective against lead leaching.
It has been recently suggested that brass fittings be treated in a very low pH copper chloride bath to reduce the rate of lead leaching from the fittings during consumer use. It was thought that this treatment would mimic the process occurring in situ over many years. However, the efficacy of this treatment has been somewhat erratic. The copper chloride concentration ranged from 1 millimolar (mM) to 100 mM, while the preferred pH was a pH of 2.0. This very low pH may unduly endanger workers' safety. During treatment, the pH increased to non-preferred ranges, becoming less effective.
Copper chloride treatments have been found to have other serious disadvantages, including: the treatments are non-specific and inefficient, resulting in high amounts of zinc leached as well as lead; insufficient amounts of lead are leached; and the treatments are corrosive, with the low pH adversely affecting the treatment facilities.
It has been discovered that immersing or otherwise exposing the lead-containing surfaces of brass plumbing fittings to a bath of 1 mM copper (cupric) acetate (CuAc.sub.2) to 100 mM CuAc.sub.2, for a period of at least about five minutes, will effectively, efficiently and consistently reduce the lead leached into water to substantially less than the normalized 11 .mu.g called for by the NSF International consensus standard of December 1994.